Sorcery level affects the power of your spells (roughly double for every 10 levels according to the manual). I don't see anything in there about what a higher level in the spell category itself will do other than the penalty for being in another guild (ie: the half-level deal with learning new spells). If that's the only thing, it would mean that you couldn't learn level 7 spells if you change to a guild that doesn't specialize in that category of magic no matter what you did because you could only go up to level 12. I did train these up to level 12, but I haven't been checking the spell costs, so I couldn't say if they've come down over time or not.